Analysts claim the leak - which appears to come from Russia's state-owned Channel One - may have been on the orders of President Vladimir Putin in revenge against the BBC

The BBC has launched an investigation into the leak amid claims it could have been to retaliate against the broadcaster over its decision to expand its Russian language output.

The Kremlin is believed to have been angered by plans to expand the BBC World Service’s Russian language output, and also threatened the corporation, believing it to be involved in NatWest closing bank accounts linked to Russia’s English language broadcaster RT.

The leaked episode appears to be from Russia's state-owned Channel One, featuring a three-minute continuity message from the channel.

Ben Nimmo, information defence fellow at the Atlantic Council thinktank, told The Independent: "When you’ve eliminated the impossible, whatever remains, however improbable, must be the truth’.

“There appears to be no profit motive, no benefit to the broadcaster from doing this. What remains is a political motive. The most obvious explanation is that this is punitive.”

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Lilit Gevorgyan, senior economist at IHS Global Insight, told The Telegraph: “There has been a pattern of Russia making shortterm tactical gains at the expense of long-term strategic losses, so we cannot entirely exclude the possibility that this is a deliberate act.”

The BBC is understood to have upgraded its file security for its TV output after scripts and unfinished footage from Doctor Who were leaked from the Miami office of BBC Worldwide in 2014.

The final of Sherlock drew its lowest-ever overnight UK ratings on Sunday night, with around 5.9m tuning in to watch Benedict Cumberbatch's sleuth in The Final Problem - down from the 8.1m who watched the first episode on New Year’s Day.

The second episode has now been watched by a consolidated audience of 9.5m which is up 3.5m on the overnight audience.

More than 11 million viewers watched the episode broadcast on New Year’s Day once consolidated figures were taken into account, making it the biggest audience of the festive period.

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